1. Field of the Invention
The present invention is directed to hardfacing materials and methods for applying them.
2. Description of Related Art
Pieces, parts and components in a variety of machines, devices, and apparatuses are subjected to abrasive and erosive wear, often in a corrosive media. For years attempts have been made to solve these problems by applying hard wear-resistant layers to these components, e.g. means as brazing inserts of hard materials to critical areas or by applying hard coatings to the surfaces by detonation gun, plasma arc spraying (transferred and non-transferred), welding (gas or electric arc), electroplating, sputtering or ion plating. All of these suffer from certain limitations. The use of inserts is expensive and is not compatible with most application geometries. Detonation gun coating provide some of the best coatings, but are limited in the thicknesses that can be applied, the geometries that can be addressed, and may be relatively expensive for some high volume applications. Sputtering and ion plating are expensive. Electroplating can be limited in the materials that can be effectively used, chromium probably being the hardest of those used for wear resistance.
A variety of welding techniques are commonly used to apply the general class of hardfacing compositions. These materials have good wear resistance and can be applied in thick layers. Hardfacing material fused on the surface of a substrate can involve a significant amount of dilution with the substrate metal. This is a result of mixing of molten hardfacing and the surface of the substrate, which can reduce the wear resistance of the deposit and waste material. Often the control of the process is limited and very rough surfaces are created, which are removed by grinding before the component can be placed in service. Grinding costs can add additional expense.
In several prior art hardfacing techniques, a series of cracks result in the hardfacing; e.g. see U.S. Pat. Nos. 3,494,749; 5,224,559; 3,402,459; and 3,407,478, all incorporated fully herein for all purposes. This cracking is undesirable since the crack tips can become stress concentrators, and the crack can propagate through the material. Cracks can collect foreign materials and become a potential corrosion site. To deal with these problems, parts have been made thicker and heavier to withstand the cracking. In some cases parts have been over-designed to compensate for the existence of cracks or to try to eliminate crack propagation altogether. Some parts have been used or allowed to run without the benefits of surfacing because dealing with cracks is too time consuming and expensive.
Often hardfacing materials used for surfacing by welding are relatively higher alloyed than the materials to which they are applied. High alloy steel is an iron-carbon alloy containing at least 5% by weight of additional elements. Higher alloyed steel has 4 to 8 times by weight additional elements. The difference in alloy content can affect the coefficient of thermal expansion in such a manner that surface cracking results. This cracking is caused by the difference in the coefficient of thermal expansion of the surfacing and the coefficient of thermal expansion of the substrate. Post-weld cracks can form initially upon solidification from the molten weld as a stress relief mechanism; or they can form when stress caused by differences in coefficient of thermal expansion exceeds the tensile strength of the material. In many cases an initial surfacing contains no cracks, but subsequent heating and cooling cycles promote fatigue cracking. In many cases the asserted stress relief function of unwanted cracks is of minimal value.